1. Field of the Invention
The present invention relates to a method of growing a silicon crystal in a liquid phase. A silicon crystal produced by the method of the present invention can be used in silicon devices having a large area such as solar cells and picture element driving circuits for liquid crystal display devices.
2. Related Background Art
Solar cells are prevailing as electric power sources which are systematically linked with driving power sources for various kinds of appliances and commercial line power. It is desirable to manufacture solar cells at low cost. For example, it is desired to produce solar cells on inexpensive substrates at a low cost. Silicon is generally used as a semiconductor for composing solar cells. Single crystalline silicon is extremely excellent from a viewpoint of efficiency of converting light energy into electric power, that is, photoelectric conversion efficiency. From the viewpoints of enlargement of area and reduction of manufacturing cost, on the other hand, amorphous silicon is advantageous. In recent years, polycrystalline silicon has been used for the purpose of obtaining a cost as low as that of amorphous silicon and a photoelectric conversion efficiency as high as that of single crystalline silicon.
However, it cannot be said that the expensive crystalline materials are sufficiently utilized by a method which is conventionally adopted to manufacture silicon devices using single crystalline silicon or polycrystalline silicon since the method is configured to slice a lump crystal to form plate-like substrates and is hardly capable of preparing substrates which have thicknesses of 0.3 mm or smaller, thereby allowing the substrates to have thicknesses larger than a thickness (20 xcexcm to 50 xcexcm) generally required to absorb incident rays. Furthermore, there has recently been proposed the spin method of forming a silicon sheet by flowing drops of melted silicon into a template. However, a silicon sheet formed by this method has a quality insufficient for use as a semiconductor and cannot provide a photoelectric conversion efficiency which is so high as that in the case of using a general crystalline silicon.
There has been proposed and actually applied to trial production of a solar cell under the circumstances described above, an idea of growing on an inexpensive substrate a silicon crystal of a good quality until it has a required and sufficient thickness and forming an active region (for example, a photoelectric conversion region) thereon. Moreover, there has been proposed an idea of growing a silicon crystal epitaxially on a substrate of a good quality and then peeling off the silicon crystal and reusing the substrate.
On a premise that large area devices such as solar cells are to be produced in mass, however, it is not so easy to grow a silicon crystal until it has a thickness required for absorbing incident rays. A silicon crystal of a good quality is generally grown by the thermal CVD method of thermally decomposing a raw material gas such as silane chloride. In order to grow a single crystal at a high rate on the order of 1 xcexcm/minute in particular, it is typical to use the so-called epitaxial growing furnace. However, such a growing furnace is not only unsuited to mass production since it can treat 10 wafers at most at one batch, but also requires a high raw material cost since it utilizes a raw material gas at a low efficiency. Though it is possible to treat 100 or more wafers at one batch by utilizing the so-called low pressure CVD furnace, this furnace also provides a crystal insufficient in quality and allows the crystal to grow at a rate only on the order of 0.01 xcexcm/minute, thereby being low in productivity.
As another method of growing a silicon crystal, there is known a liquid phase growing method of supersaturating a liquid metal solution in which silicon is dissolved and allowing a crystal to deposit from the solution onto a substrate. This liquid phase growing method is capable of growing a crystal of a high quality at a high rate on the order of 1 xcexcm/minute and treating 100 or more wafers at one batch, thereby being suited to mass production. However, the liquid phase growing method is not generally used for growing silicon and has some technical problems to be solved though it widely prevails as a method of growing compound semiconductors.
One important problem lies in selection of a metal which is to be used as a solvent. It is desirable that a metal to be used for this purpose has a solubility for silicon which is as high as possible and can hardly be incorporated into deposited silicon. Furthermore, a metal having a lower melting point and a lower vapor pressure can be handled easier. Tin is used most generally as a solvent for silicon. Tin can be handled relatively easily since it has a low melting point and a relatively high solubility for silicon. It has been considered that tin is a preferable solvent since tin and silicon belong to Group IV of the periodic table, and tin is inactive as a dopant even when it is incorporated into deposited silicon.
However, the inventors have recently found that tin is incorporated into silicon in a relatively large amount when growth conditions (in particular, a growth temperature) are inadequate, thereby deforming a lattice of a silicon crystal and adversely affecting electric characteristics of a semiconductor probably due to the atomic size of tin which is very different from that of silicon though they are atoms belonging to Group IV. From this viewpoint, there is posed a doubt in the aptitude of tin as a solvent which is used to grow a crystal for a solar cell with high efficiency.
In addition to tin, elements such as gallium, indium and aluminum which belong to Group III can be mentioned as metals which are usable as solvents. Gallium and indium, in particular, having a low melting point can be handled easily. Since gallium is extremely expensive, indium is hopeful for use as a practical melt. However, indium posed a problem which is described later in control by introducing dopant a conductivity type of a silicon crystal which is grown using an indium melt. There are known examples wherein gallium is used as p-type dopant in combination with an indium melt (G. F. Zheng et al.: Solar Energy Materials and Solar Cells. 40 (1996) 231-238). Though gallium is usable at relatively low concentrations, it cannot be used for doping at high concentrations since a solid of gallium can be dissolved into silicon at concentrations within a relatively low solubility and is extremely expensive. On the other hand, examples which use n-type dopants in combination with indium melts are disclosed by Japanese Patent Application Laid-Open Nos. 9-183695 and 9-183696.
Boron and aluminum are generally used as p-type dopants, whereas phosphorus and arsenic are often used as n-type dopants. It is therefore conceivable to use these dopants for growing silicon crystals in liquid phase with the indium melt. In practice, however, problems were posed in conductivity types or reproducibility of conductivities of grown silicon crystals in certain cases. Furthermore, it is feared that a metal of Group III such as indium which is originally active by itself as a dopant may control a crystal to a strong p-type when incorporated into silicon and may be incapable of controlling it to pxe2x88x92-type or n-type.
The problems described above make it still impossible to judge whether or not the liquid phase method has a true aptitude for growth of silicon crystals on scales of mass production and whether or not solar cells utilizing thin films of silicon crystals have practical utility.
Thin films of silicon crystals are also used as devices for driving picture elements of liquid crystal displays and so on. Progress made in mass communication media have produced increasing demands for a display having a larger screen and capable of more minutely driving at a higher speed. Though the TFTs (thin film transistors) of amorphous silicon have hitherto been utilized as a driving circuit for picture elements to cope with the demands for a display having a larger screen, amorphous silicon can no longer meet the demands for a display which can be more minutely driven at a higher speed, and TFTs of polycrystalline silicon are coming into use. In addition, there has been increasing demand for polycrystalline silicon which has higher carrier mobility and other characteristics.
The liquid phase growing method is also suited for growing such crystalline silicon of a high quality on a large substrate such as a glass plate. Though use of a glass plate or the like makes it unallowable to heat a solution to a high temperature, it is possible to grow a crystal of a good quality by using indium as a solvent. Though it is impossible to grow a thick crystal at a low growth temperature which lowers a solubility of silicon into indium, there is no problem in formation of a crystal to be used as a TFT having a thickness of the order of 0.1 to 0.5 xcexcm which is far smaller than that of a solar cell. When indium is used as a solvent for production of a TFT, a problem related to reproducibility may be posed. Therefore, a concentration of a dopant must be precisely controlled in order to enhance reproducibility of characteristics of the TFT. In formation of a film having a large area, an ununiform distribution of a dopant concentration is not preferable as it produces an ununiform distribution of characteristics of TFT, thereby producing variations in image density on a display device. In certain cases where indium was used as a dopant, it was impossible to sufficiently prevent the dopant from being distributed ununiformly on surfaces.
The present invention has been achieved in view of the current circumstances described above, and an object of the present invention is to provide a method of precisely controlling a dopant to be incorporated into crystalline silicon which is grown in a liquid phase using indium as a solvent, thereby enabling mass production of solar cells having a high efficiency and a light weight as well as driving circuits for a high precision and high speed display having a large area.
The present invention therefore provides a method of growing a silicon crystal, which comprises using a melt prepared by dissolving a solid of silicon containing a dopant at a predetermined concentration into liquid indium. Furthermore, the present invention provides a method of growing a silicon crystal, which comprises using a melt prepared by dissolving a solid of indium containing a dopant at a predetermined concentration into liquid indium.
Moreover, the present invention provides a method of producing a solar cell, which comprises the steps: preparing a melt by dissolving a solid of silicon containing a dopant at a predetermined concentration into liquid indium; forming a first silicon layer of a first conductivity type on a substrate by bringing the substrate into contact with the melt; and forming a second silicon layer of a second conductivity type on the first silicon layer of the first conductivity type.
In addition, the present invention provides a method of producing a solar cell, which comprises the steps of: preparing a melt by dissolving a solid of indium containing a dopant at a predetermined concentration into liquid indium and further dissolving silicon into the melt; forming a first silicon layer of a first conductivity type on a substrate by bringing the substrate into contact with the melt; and forming a second silicon layer of a second conductivity type on the first silicon layer of the first conductivity type.